1. Field
The present invention relates to fuel cell units, control methods for fuel cell units, and information processing apparatuses. In particular, the present invention relates to a fuel cell unit for performing setup processing, a method for controlling the fuel cell unit and an information processing apparatus connected with the fuel cell unit.
2. Description of the Related Art
Currently, for example, lithium-ion batteries are used as secondary batteries, which are one type of power supply sources, for information processing apparatuses. One feature of the secondary batteries is that they can be repeatedly used upon being charged with, for example, a commercial power supply, as opposed to disposable primary batteries.
The lithium-ion batteries, however, requires a commercial power supply for charging, since they are secondary batteries.
In conjunction with remarkable improvements in the features and performance of information processing apparatuses in recent years, the power consumption of the information processing apparatuses are also on an upward trend. Thus, it is desired that the density of energy, i.e., the amount of energy output per unit volume or unit mass, provided by lithium ion batteries that supply power to the information processing apparatuses is increased. It is, however, difficult to expect a remarkable increase under the current situation.
In theory, the energy density of a fuel cell is said to be ten times as much as that of a lithium ion battery (for example, refer to “Fuel Cell 2004 (Nenryou-Denchi 2004)” Nikkei Business Publications, Inc., pp. 49-50 and pp. 64, October 2003, hereinafter referred to as Non Patent Document 1). This indicates that, when the fuel cell has the same volume or mass as the lithium battery, the fuel cell has the potential to supply power for a (e.g., ten times) longer period of time. Further, that also indicates that, when both have the same power-supply time, the fuel cell has the potential to be reduced in size and weight as compared to the lithium ion battery.
Further, with a fuel cell, fuel such as methanol can be encapsulated into a small container, and the replacement of the small package as a unit can eliminate the need for charging that uses an external power source. Thus, compared to a case a lithium-ion battery is used to supply power to an information processing apparatus at a place where no AC power-supply facility is available, the use of a fuel cell allows the information processing apparatus to operate for a long period.
Additionally, when an information processing apparatus (e.g., a notebook personal computer) using a lithium-ion battery is used for a long period of time, it is difficult for the information processing apparatus to operate on power supplied from the lithium-ion battery for a long period of time. Consequently, the use of the information processing apparatus is limited to an environment where power supply with an AC power source is available. In contrast, the use of a fuel cell to supply power to the information processing apparatus allows the information processing apparatus to operate for a longer period of time, compared to the case of using a lithium-ion battery, and can also provide the advantage of eliminating the above-noted limitation.
In view of the foregoing situation, fuel cells aimed to supply power to information processing apparatuses are under research and development. For example, relevant technologies are disclosed in Japanese Patent Application Publication (KOKAI) No. 2003-142137, Japanese Patent Application Publication (KOKAI) No. 2003-86192, and Japanese Patent Application Publication (KOKAI) No. 2002-169629.
Various types of fuel cell systems are available (e.g., refer to “Everything of Fuel Cell (Nenryoudenchi-no-subete),” Hironosuke Ikeda, Nippon Jitsugyo Publishing Co., Ltd., August 2001, hereinafter referred to as Non Patent Document 2). However, when compactness, lightweight, and the ease of handling of the fuel cell are considered, for example, a direct methanol fuel cell (DMFC) is suitable for information processing apparatuses. This fuel cell system uses methanol as a fuel and the methanol is directly introduced into a fuel cell electrode without being converted into hydrogen.
For the direct methanol fuel cell, the concentration of methanol introduced into the fuel electrode is important. A high concentration leads to decreased power generation efficiency, which makes it impossible to provide satisfactory performance. This is due to a phenomenon (called a crossover phenomenon) in which part of methanol, which is used as a fuel, passes through an electrolyte membrane (specifically, a solid polymer electrolyte membrane) sandwiched between the fuel electrode (a negative electrode) and an air electrode (a positive electrode). The crossover phenomenon becomes more pronounced for high-concentration methanol and is attenuated when low-concentration methanol is introduced into the fuel electrode.
On the other hand, when low-concentration methanol is used as a fuel, high performance can readily be ensured but the volume of the fuel is increased (e.g., by a factor of ten) compared to a case in which high-concentration methanol is used. Thus, the fuel container (i.e., the fuel cartridge) becomes large.
Accordingly, miniaturization can be achieved by encapsulating high-concentration methanol into the fuel cartridge. In addition, the crossover phenomenon can be reduced by causing small pumps, values, and so on to circulate water produced during power generation and reducing the concentration of the high-concentration methanol by dilution before the methanol is introduced into the fuel electrode. This system can also improve the power generation efficiency. Hereinafter, those pumps, valves, and so on for circulation will be referred to as “auxiliary sections” and such a system for circulation will be referred to as a “dilution circulation system”.
Such an approach (as disclosed in Non Patent Document 1) can achieve a compact, lightweight fuel-cell unit having high power-generation efficiency.
Before problems to be solved by the invention are described, the principle of operation of a fuel cell will be briefly described first. Since the principle of operation is already described in detail in known documents (e.g., Non Patent Document 1 mentioned above), an overview of the principle will now be described.
FIG. 1 illustrates the principle of operation of a direct methanol fuel cell (DMFC) 5. In the DMFC 5, an electrolyte membrane 1 is arranged at the center and is sandwiched by a fuel electrode (a negative electrode) 2 and an air electrode (a positive electrode) 3 from two opposite sides.
When a methanol-water solution is introduced into one end of the fuel electrode 2 of the DMFC 5, oxidation reaction of methanol occurs at the fuel electrode 2. As a result, electrons (e−), hydrogen ions (H+), and carbon dioxide (CO2) are generated. The hydrogen ions (H+) pass through the electrolyte membrane 1 to reach the air electrode 3. The carbon dioxide (CO2) is exhausted from another end of the fuel electrode 2.
The electrons (e−) circulate from the fuel electrode 2 to the air electrode 3 via a load 4. The flow of the electrons enables power to be supplied to an external apparatus. At the air electrode 3, oxygen (O2) in the air that is externally introduced reacts with the hydrogen ions (H+) that has passed through the electrolyte membrane 1 and the electrons (e−) that have circulated via the load 4, so that water H2O (water vapor) is produced.
FIG. 1 illustrates only one unit of the fuel cell structure, and, in practice, a plurality of DMFCs 5 are stacked to provide a predetermined voltage and current. The stack of the DMFCs 5 is called a “DMFC stack”.
In direct methanol fuel cells, a solid polymer electrolyte membrane is typically used as the electrolyte membrane 1. It is well known to those skilled in the art, as discussed in Japanese Patent Application Publication (KOKAI) No. 2003-86192, that when the solid polymer electrolyte membrane is dried, for example, after being left unused for a long period, the membrane does not serve as an electrolyte. That is, the solid polymer electrolyte membrane displays conductivity with respect to hydrogen ions (H+) when hydrated, but becomes an isolator with respect to hydrogen ions (H+) when dried and thus does not serve as an electrolyte.
Thus, when the fuel cell unit is used for the first time after shipment or is left unused for a long period after the stopping of power generation, the power-generation capability of the fuel cell unit may decrease due to the dried solid polymer electrolyte membrane.
In order to prevent the decrease in power-generation capability, special processing called setup processing may be performed. The setup processing refers to hydration processing for sufficiently hydrating the solid polymer electrolyte membrane when the fuel cell unit is used for the first time or is left unused for a long period of time, for example, for one year. In the setup processing, moisture is introduced into the dried fuel electrode by operating the liquid feed pump(s) of the auxiliary sections for a certain period of time.
The setup processing is aimed to hydrate the solid polymer electrolyte member but is not to generate power. Also, the processing is not necessarily required every time the fuel cell unit is used. Thus, a specific sequence, which is different from an ordinary operation sequence, can be set for the setup processing. This allows the processing for hydrating the solid polymer electrolyte membrane to be efficiently performed only when necessary.